Portable communication devices, such as cellular telephones, personal digital assistants (PDAs), WiFi transceivers, and other communication devices transmit and receive communication signal at various frequencies that correspond to different communication bands and at varying power levels. A power amplifier module, generally comprising one or more amplification stages, is used to transmit the communication signals. A radio frequency (RF) power amplifier system may include multiple amplification stages, and, in some applications, multiple amplification paths. The efficiency of the power amplifier system is generally determined by a number of factors, and to a large degree, determines the amount of power consumed by the power amplifier system.
Various ways of measuring and determining the efficiency of the power amplifier are available. One measure of power amplifier efficiency is referred to as “power added efficiency,” abbreviated as PAE. The PAE of a power amplifier is dependent upon a number of factors including, but not limited to, the number of power amplification paths if the power amplifier system is a dual mode power amplifier, the load impedance at the output of the power amplifier path or paths, impedance matching between multiple stages, and other factors. In a power amplifier that is capable of transmitting at multiple power levels, one method to improve the PAE at lower power is to reduce the bias voltage of the collector terminal (or drain terminal, if the power amplifier is implemented as a field effect transistor (FET) device) of the power amplifier. This can be done in conjunction with adjusting the bias voltage of the base terminal (or gate terminal of a FET device) of the power amplifier. The bias voltage for the collector is typically provided by a voltage converter that can provide a continuous range of output values.
Unfortunately, a voltage converter that is capable of providing a continuous range of output values usually contains lossy inductive elements, and can be overly complex in applications where a limited number of discrete output values may suffice.
Therefore, it is desirable to maximize the PAE of a power amplifier system over a range of power levels, without sacrificing the linearity and performance of the power amplifier system over the range of power output levels.